1. Field
The present invention relates to an analog-to-digital converter (ADC), and more particularly, to a sigma-delta ADC.
2. Related Art
An ADC is a device or a circuit which converts an analog signal into a digital signal and is usually used in CMOS image sensors. A CMOS image sensor is usually used in portable devices such as mobile phones and digital cameras and converts an optical signal into an electrical signal. The conversion occurs at a pixel of the CMOS image sensor including a photodiode and a readout circuit.
In an image sensor, a pixel array includes a plurality of pixels arranged in a two-dimensional matrix and each pixel outputs an image signal from light energy. The pixel integrates a photo-charge corresponding to the amount of light input through a photodiode and outputs a pixel signal in a form of analog current based on the integrated charges. The pixel signal is transmitted to a readout circuit. The readout circuit converts the analog signal into a digital signal.
A single ADC circuit is provided for each of the columns of the pixel array to convert an analog signal output from the pixel array into a digital signal in the image sensor. Accordingly, as many ADC circuits as the number of columns of the pixel array are needed, which is a big burden in terms of power consumption and area.